I recently made a bunch of 6mm-scale hills and I didn't want to have to glue sand on them and then spray undercoat them; too much effort and too much risk that the spray would dissolve the foam. But I also didn't want to pay $60 for a 4L tin of textured paint.
The solution? Berger True Grip.
This is the stuff you mix into outdoor paint for use around pools and it gives a rough surface to stop you from slipping over. It comes in 300g jars that cost about $8 and gives you enough to make up 4L of paint (i.e. heaps and heaps). Now all I needed was a reasonably-sized tin of paint to mix it into.
Me: "Do you guys sell sample pots?"
Bunnings Guy: "Yup."
Me: "What sort of finish do they have?"
Bunnings Guy: "They're all low-sheen for interior walls."
Me: "How much?"
Bunnings Guy: "$8 for 250ml."
Me (handing over my carefully-selected colour-matched paint chip): "Brilliant. Give me a pot of this colour."
All that was left to do was mix them together. The ratio is 75g per litre of paint, so for my little pot I needed 18.75g. There was no way I could ever measure that accurately so I decided to just keep adding spoonfuls and stirring them in until I got the consistency I wanted. This worked really well although it took more than I thought to get there (about ten spoonfuls), and I had to stir each one in thoroughly before I added the next. I also ignored the instruction which said "remove 10% of the paint while you mix it up and add it back in afterwards." This is just to prevent spillage and there was enough space in the sample pot that I didn't need to worry about it.
The final result was awesome - one coat was enough for each hill and gave them a lovely textured surface. You could definitely vary the amount of texture in the pot by adding more or less of the True Grip.
So for $16 I got a tin of water-based textured paint in exactly the right colour. I only used half of it in doing all ten of my hills, and I've got enough True Grip left to make up at least another ten sample pots worth of paint. This is the technique I will use on all of my terrain projects in future.